Transport API and dataset for UK.
Transport for UK API is a free Transportation API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Details from developer.transportapi.com.

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Washington Metro transport API
The Transport for UK API is provided by TransportAPI (developer.transportapi.com).
Yes. The Transport for UK API is listed in the Transportation category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Transport for UK API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Transport for UK API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Transport for UK API is served over HTTPS, so it's safe to use in production.